System and method for controlling operation of an electrosurgical system

ABSTRACT

An electrosurgical system including or connected to an output circuitry comprising an electrosurgical device and an electrical cable is modeled during a cable interrogation phase using a transfer matrix in order to determine a leakage capacitance in the electrosurgical system. After the leakage capacitance is assigned or set to a virtual capacitor in the transfer matrix, an output parameter of the electrosurgical system, such as output voltage, output current, output impedance or output electrical power, may be determined by applying an actual input voltage to the output circuitry and measuring a resulting input current, and multiplying the input voltage and measured current by the transfer matrix.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 14/851,310, filed Sep. 11, 2015, which claims the benefit ofand priority to U.S. Provisional Patent Application No. 62/052,062,filed on Sep. 18, 2014 and U.S. Provisional Patent Application No62/052,046, filed on Sep. 18, 2014. This application is related to U.S.patent application Ser. No. 14/851,258, filed on Sep. 11, 2015. Theentire contents of each of the foregoing applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The present disclosure relates to electrosurgical systems. Moreparticularly, the present disclosure is directed to methods forcontrolling operation of an electrosurgical system duringelectrosurgical procedures, and to an electrosurgical system that usesthe methods.

BACKGROUND

Damaged tissues are sometimes treated using an energy delivering system.Various types of therapeutic energies (e.g., electrical, ultrasonic,microwave, cryogenic, heat, laser, etc.) used in tissue treatment, whichare known in the art, may be applied to treat a tissue. Electrosurgeryis a tissue treating technique involving delivering of high radiofrequency (“RF”) electrical energy (e.g., 1-70 watts in auto bipolarelectrosurgical systems, 1-300 watts in monopolar electrosurgicalsystems). Electrosurgery treatment is rendered by an electrosurgicaldevice (e.g., electrosurgical forceps).

Electrosurgical systems conventionally monitor an electrical voltage andan electrical current in order to ‘remotely’ evaluate the impedance atthe electrosurgical device. Evaluation of the impedance at theelectrosurgical device enables an electrosurgical system to detectwhether the electrosurgical system is in an ‘open circuit’ state inwhich the electrosurgical device does not touch the treated site, or ina ‘closed circuit’ state in which the electrosurgical device touches thetreated site. Distinguishing between these two states enables theelectrosurgical system to output (generate) therapeutic energy only whenthe electrosurgical device touches the treated site.

When the surgical device touches a tissue, the impedance evaluated bythe electrosurgical system is relatively low (a few tenths of ohms toseveral thousands of ohms). When the surgical device is detached from atreated tissue, the impedance measured by the electrosurgical systemshould ideally be infinite (or practically very high; e.g., in the orderof tenths of mega ohms). Should this be the case, an impedance gapbetween the open circuit state and the closed circuit state would havebeen very large, which would enable the electrosurgical system todistinguish between the two states easily and reliably. However, inpractice, the electrosurgical system's periphery, which may include, forexample, cable(s), adapter(s), connector(s), surgical device(s), etc.,includes parasitic (leakage) impedances that detrimentally affect theimpedance gap, that is, they narrow the gap. Depending on the electricalcharacteristics of the electrosurgical system's periphery, the impedanceundesirably imposed by it may vary, for example, from tenths of ohms totenths of kilo ohms.

The parasitic (e.g., leakage) impedance imposed on the electrosurgicalsystem by its periphery poses a problem which is that theelectrosurgical system, which monitors the impedance duringelectrosurgical procedures, might erroneously interpret a parasiticimpedance as an impedance that results from a tissue contact even incases where the electrosurgical device does not touch the tissue. As aresult of this, the electrosurgical system might erroneously continue todeliver therapeutic energy to the electrosurgical device, or resumedelivery of the therapeutic energy, even though the device (e.g.,forceps) is not touching the surgical site. Holding the treatment deviceby a surgeon may also add parasitic impedance, which exacerbates theproblem of misinterpretation of the impedance evaluated by theelectrosurgical system. (The surgeon may hold the device prior totreatment and then during treatment, and, occasionally, s/he may detachit from the treated tissue and, therefore, a parasitic impedance causedby the surgeon might change during the electrosurgical process as well.)

Because, conventionally, the electrical voltage and current that theelectrosurgical system uses to evaluate the impedance at theelectrosurgical device, hence the device-tissue contact degree, do notgenuinely represent the real impedance at the electrosurgical device(due to the aforesaid parasitic/leakage capacitances), not only thatdistinguishing between the open circuit state and the closed circuitstate might not be reliable, the electrosurgical system mightdeliver/output therapeutic/treatment energy to the tissue which is nottherapeutically optimal.

It would be beneficial to have a method and system that enable reliableoperation of an electrosurgical system despite unknown impedance changesin the electrosurgical system during electrosurgical procedures.

SUMMARY

A method of operating an electrosurgical system, which may be connectedto an output circuitry including an electrical cable connected to anelectrosurgical device, may include defining a transfer matrix thatelectrically represents the output circuitry and includes a virtualcapacitor (Cvirtual) to represent a leakage capacitance (Clkg) in theoutput circuitry. The method may include assigning or setting, to thevirtual capacitor, a capacitance value that may represent the leakagecapacitance. The capacitance value may be known in advance (e.g., it maybe read from a device selected from the group consisting of RFID tag andbarcode, and set to Cvirtual automatically, or it may be set manually,by receiving an input from a user.). The capacitance value may not beknown in advance, in which case it may be determined by sweeping thevalue of the virtual capacitor to determine an optimal capacitance,which is a virtual/theoretical capacitance that best represents theleakage capacitance. (By ‘a virtual/theoretical capacitance that bestrepresents the leakage capacitance’ is meant a virtual/theoreticalcapacitance whose value may be identical to, or approximately the sameas, the value of the leakage capacitance.)

The method may, during an electrosurgical procedure, also include (i)applying, by the electrosurgical system, an input voltage (Vin) to an(e.g., input end of) the electrical cable, and measuring an inputcurrent (Iin) in the electrical cable in response to the input voltage,Vin; (ii) calculating an output impedance (Zout) of the output circuitryfrom the input voltage (Vin) and the input current (Iin) using thetransfer matrix, and (iii) comparing the output impedance (Zout) to animpedance threshold value. A determination as to a current, or next,operation mode of the electrosurgical system may be based on thecomparison result.

Calculating the output impedance (Zout) of the electrical cable mayinclude calculating, by using the transfer matrix, an output voltage(Vout) and an output current (Iout) for the output circuitry from theinput voltage (Vin) and from the input current (lin). The output voltage(Vout) and the output current (Iout) calculated for the output circuitrymay be used to calculate an electrical power (Pout) that is delivered toa bodily organ or tissue from the electrosurgical system via theelectrosurgical device.

Assigning the optimal capacitance value to the virtual capacitor(Cvirtual) may include, during a cable interrogation phase, (i) changingthe value of the virtual capacitor across a series of capacitance valuesand calculating, by using the transfer matrix representative of theoutput circuitry, a series of output impedances respectivelycorresponding to the series of capacitance values; (ii) deriving amaximum output impedance (Zmax) from the series of output impedances;(iii) determining an optimal capacitance value for the virtual capacitorfor which the output impedance is maximal; and (iv) assigning theoptimal capacitance value to the virtual capacitor. The value of thevirtual capacitor (Cvirtual) may be changed (swept) according to acapacitance interval, for example it may be swept within a capacitancerange 50 pF-600 pF. (Other capacitance ranges may be used.)

Assigning the optimal capacitance value to the virtual capacitor mayinclude disconnecting the electrosurgical device from the electricalcable, then applying, by the electrosurgical system, an input voltage(Vin) to the electrical cable and measuring an input current (Iin) inthe electrical cable in response to the applied input voltage (Vin) and,for each value of the virtual capacitor, multiplying the input voltage(Vin) and the input current (Iin) by the transfer matrix to obtain anoutput voltage (Vout) and an output current (Iout) for the outputcircuitry, and calculating an output impedance (Zout) of the outputcircuitry from the output voltage (Vout) and the output current (Iout).The maximum output impedance (Zmax) may be selected from the series ofoutput impedances, or it may be interpolated from the series of outputimpedances.

Determining an operation mode of the electrosurgical system may includedetermining whether the electrosurgical device is connected to, ordisconnected from, the electrical cable, and/or determining whether theelectrosurgical device touches a bodily organ or tissue, and/or theelectrical cable is connected to, or disconnected from, theelectrosurgical system, and/or determining, based on the comparisonresult, whether the output circuitry is in an “open circuit” state inwhich the electrosurgical device does not touch a bodily organ ortissue, or in a “closed circuit” state in which the electrosurgicaldevice touches the bodily organ or tissue.

The method as in claim 1, wherein the output circuitry of theelectrosurgical system comprises any of: (i) the electrical cableconnected to the electrosurgical system, (ii) the electrosurgical devicetouches a bodily organ or tissue, (iii) an adapter connecting theelectrical cable to the electrosurgical system, (iv) an adapterconnecting the electrosurgical device to the cable, and (v) theelectrosurgical device is held by a user, and including any combinationthereof.

According to another embodiment, a method of operating anelectrosurgical system, which includes a signal generator and an outputcircuitry comprising an electrical cable connecting an electrosurgicaldevice to the electrosurgical system, may include (i) applying an inputvoltage (Vin) to the electrical cable and measuring an input current(Iin) in the electrical cable in response to the input voltage, Vin,(ii) calculating an electrical parameter of the output circuitry fromthe input voltage (Vin) and the input current (Iin) and by using atransfer matrix electrically representing the output circuitry. Anoperation mode for the electrosurgical system, or to which theelectrosurgical system is to transition, may be determined based on thevalue of the electrical parameter.

The transfer matrix may include a virtual capacitor (Cvirtual), and themethod may include assigning or setting to the virtual capacitor(Cvirtual) a capacitance value that represents a leakage capacitance(Clkg) in the output circuitry. The leakage capacitance (Clkg) mayinclude one or more of: a leakage capacitance due to the cable, aleakage capacitance due to the electrosurgical device, a leakagecapacitance due to a connector connecting the cable to theelectrosurgical system, a leakage capacitance due to a connectorconnecting the cable to the electrosurgical device, and a leakagecapacitance due to a subject (e.g., surgeon, technician, etc.) touchingthe electrosurgical device.

Assigning the capacitance value representative of the leakagecapacitance (Clkg) to the virtual capacitor (Cvirtual) may includeassigning or setting a value of the leakage capacitance (Clkg) to thevirtual capacitor (Cvirtual). Alternatively, assigning the capacitancevalue representing the leakage capacitance (Clkg) to the virtualcapacitor (Cvirtual) may include, during a cable interrogation phase,(i) changing the value of the virtual capacitor (Cvirtual) of thetransfer matrix across a series of capacitance values and calculating,by using the transfer matrix, for the output circuitry, a series ofoutput impedances respectively corresponding to the series ofcapacitance values; (ii) deriving a maximum output impedance (Zmax) fromthe series of output impedances; (iii) determining an optimalcapacitance value for which the output impedance is maximal (Zmax); and(iv) assigning the optimal capacitance value to the virtual capacitor(Cvirtual).

The operation mode may be selected from the group consisting of: (i)delivering, by/from the signal generator, via the electrosurgicaldevice, a therapeutic energy to a treated site, (ii) adjusting anelectrical parameter of the signal generator when or while thetherapeutic energy is delivered to the treated site, and (ii) refrainingfrom delivering therapeutic energy to the treated site.

The electrical parameter may be an output impedance (Zout) of the outputcircuitry, and the method may include determining, based on a value ofthe output impedance (Zout), the operation mode of or for theelectrosurgical system and/or distinguishing between an ‘open circuit’state of the output circuitry in which the electrosurgical device isdetached from a treated site, and a ‘closed circuit’ state of the outputcircuitry in which the electrosurgical device is attached to (touches)the treated site. Calculating the output impedance (Zout) of the outputcircuitry may include calculating an output voltage (Vout) and an outputcurrent (Iout) of the output circuitry by multiplying Vin and Iin by thetransfer matrix. The electrical parameter may be an output electricalpower (Pout) that is delivered or to be delivered to a treated site viathe electrosurgical device. Calculating the output electrical power mayinclude calculating an output voltage (Vout) and an output current(Iout) of the output circuitry by multiplying Vin and Iin by thetransfer matrix. The method may include controlling the outputelectrical power to provide an optimal therapeutic energy to the treatedsite during therapeutic energy delivery.

Also provided is an electrosurgical system that may include atherapeutic energy delivering generator, a controller to control thetherapeutic energy generator, and an output circuitry. The outputcircuitry may include, for example, an electrosurgical device totransfer therapeutic energy to a treated site, and an electrical cablethat may electrically connect the electrosurgical device to theelectrosurgical system.

The controller may be configured to define a transfer matrix thatelectrically represents the electrosurgical system, or the outputcircuitry. The transfer matrix may include a virtual capacitor(Cvirtual) for representing a leakage capacitance (Clkg) in theelectrosurgical system, or in the output circuitry. The controller maybe configured to assign, to the virtual capacitor (Cvirtual), acapacitance value that may represent the leakage capacitance (Clkg), andto monitor an electrical parameter of the output circuitry by (i)applying an input voltage (Vin) to the electrical cable and measuring aninput current (Iin) in the electrical cable, and (ii) calculating anelectrical parameter of the output circuitry from the input voltage(Vin) and the input current (Iin) and by using the transfer matrix. Thecontroller may be configured to determine an operation mode of or forthe electrosurgical system based on the value of the electricalparameter.

The electrical parameter may be selected from the group consisting of:output voltage of the output circuitry, output current of the outputcircuitry, output impedance and output electrical power delivered to thetreated site. The operation mode may be selected from the groupconsisting of: (i) delivering, from the signal generator, therapeuticenergy to a treated site, (ii) adjusting an electrical parameter of thesignal generator when the therapeutic energy is delivered to the treatedsite, and (ii) refraining from delivering therapeutic energy to thetreated site.

The controller may be configured to receive a capacitance value (if itis known in advance) that represents the leakage capacitance in theoutput circuitry, and to assign or set this value to the virtualcapacitance. If the capacitance value representing the leakagecapacitance is unknown, the controller may be configured to evaluate itby changing (‘sweeping’) the value of the virtual capacitor (Cvirtual)in the transfer matrix across a series of capacitance values, andcalculating, by using the transfer matrix, for the output circuitry, aseries of output impedances respectively corresponding to the series ofcapacitance values. Then, the controller may derive a maximum outputimpedance (Zmax) from the series of output impedances, and determine(e.g., calculate) an optimal capacitance value for which the outputimpedance is maximal (Zmax). Then, the controller may assign or set theoptimal capacitance value to the virtual capacitor (Cvirtual).

The controller may determine the capacitance value representing theleakage capacitance (Clkg) to the virtual capacitor (Cvirtual) by (i)changing the value of the virtual capacitor (Cvirtual) of the transfermatrix across a series of capacitance values and calculating, by usingthe transfer matrix, for the output circuitry, a series of outputimpedances respectively corresponding to the series of capacitancevalues, (ii) deriving a maximum output impedance (Zmax) from the seriesof output impedances, and (iii) determining an optimal capacitance valuefor the virtual capacitor (Cvirtual) for which the output impedance ismaximal (Zmax).

The controller may, while the electrosurgical device is detached fromthe treated site (e.g., during a cable interrogation phase), cause thesignal generator to output an input voltage (Vin) to the electricalcable, and to measure an input current (Iin) in the electrical cable inresponse to the input voltage (Vin), and, for each value of the virtualcapacitor (Cvirtual) the controller may multiply the input voltage (Vin)and the input current (Iin) by the transfer matrix to obtain an outputvoltage (Vout) and an output current (Iout) of the output circuitry.Then, the controller may calculate, for each value of the virtualcapacitor (hence for each transfer matrix having a different value ofCvirtual), an output impedance (Zout) of the output circuitry from thepertinent output voltage and output current.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary embodiments are illustrated in the accompanyingfigures with the intent that these examples not be restrictive. It willbe appreciated that for simplicity and clarity of the illustration,elements shown in the figures referenced below are not necessarily drawnto scale. Also, where considered appropriate, reference numerals may berepeated among the figures to indicate like, corresponding or analogouselements. Of the accompanying figures:

FIG. 1A shows a plot illustrating an ideal situation in the context ofauto bipolar electrosurgical systems;

FIG. 1B shows a plot illustrating realistic situations in the context ofautobipolar systems;

FIG. 2A shows a conceptual two-port network in according to an exampleembodiment;

FIG. 2B schematically illustrates a two-port network representing anelectrosurgical system according to an example embodiment;

FIG. 3 shows an impedance-capacitance plot in accordance with an exampleembodiment;

FIG. 4 is a block diagram of an electrosurgical system and setup andsetting according to an example embodiment;

FIG. 5 shows a method of operating an electrosurgical system accordingto an example embodiment;

FIG. 6 shows a method of operating an electrosurgical system accordingto another example embodiment;

FIG. 7 shows a block diagram demonstrating usage of the methodsdisclosed herein by an electrosurgical system according to an exampleembodiment; and

FIG. 8 schematically illustrates utilization of methods according to anexample embodiment.

DETAILED DESCRIPTION

The description that follows provides various details of exemplaryembodiments. However, this description is not intended to limit thescope of the claims but instead to explain various principles of theinvention and the manner of practicing it.

Although embodiments of the invention are not limited in this regard,discussions utilizing terms such as, for example, “processing,”“computing,” “calculating,” “determining”, “analyzing”, “checking”, orthe like, may refer to operation(s) and/or process(es) of a computer, acomputing system or other electronic computing device, that manipulateand/or transform data represented as physical (e.g., electronic)quantities within the computer's registers and/or memories into otherdata similarly represented as physical quantities within the computer'sregisters and/or memories or other information non-transitory storagemedium that may store instructions to perform operations and/orprocesses. Unless explicitly stated, the method embodiments describedherein are not limited to a particular order or sequence. Additionally,some of the described method embodiments or steps thereof can, forexample, occur or be performed at the same point in time.

FIG. 1A shows a plot 100 illustrating an ideal situation in the contextof an autobipolar (“ABP”) electrosurgical system. The horizontal axisindicates a state/condition of the ABP electrosurgical system; e.g.,system's output circuit is closed versus open. The vertical axisindicates an external output impedance to be sensed by the ABPelectrosurgical system at the tip of the electrosurgical device. (Thesame applies to the axes in FIG. 1B, which is described below.)

When an electrosurgical system performs an electrosurgical procedure,the electrosurgical (treatment) device (e.g., forceps) performing thetreatment may, at some instants, touch the treated bodily organ ortissue, and at other instants it may be intentionally moved away fromthe treatment site, for example, in order not to provide too much energyto the treated tissue/site, for example in order not to overheat thetreated tissue/site. (The treatment may, at times, be unintentionallymoved away from the treatment site.)

In ideal cases, when the treatment device touches the treated tissue(e.g., when the ABP system's electrical circuitry (the electrosurgicalsystem's ‘output circuitry’, or ‘output circuitry’ for short) is closedvia the tissue's impedance), the ABP system would sense a relativelysmall impedance (Z1, FIG. 1A), which is approximately the impedance ofthe tissue, and when the treatment device does not touch the treatedtissue (i.e., when the ABP system's output circuitry is open), theimpedance that the ABP system would sense should theoretically beinfinite (Z∞, FIG. 1A), or, in practice, at least in the order ofhundreds of mega ohms.

As shown in FIG. 1A, the impedance gap ΔZ1 (FIG. 1A) between Z1 (theimpedance during the ‘closed circuit’ state) the Z∞ (the impedanceduring the ‘open circuit’ state) is very large so that the two distinctimpedance states can be distinguished easily. Since the ABP systemshould stop delivering therapeutic RF energy when the treatment deviceis moved away from the body, the ability to reliably distinguish betweenthe two impedance states (i.e., Z1 versus Z∞) is prerequisite to safereliable and efficient operation of the electrosurgical system. However,in practice, the impedance gap is far from being ideal, in part due tothe parasitic capacitances existing in the output circuitry of theelectrosurgical system (and also in the electrosurgical system itself),as demonstrated in FIG. 1B, which is described below.

FIG. 1B shows a plot 110 illustrating example situations in the contextof an ABP system. When the treatment device (e.g., forceps) touches atreated tissue, see device condition 120; i.e., when the ABP system'soutput circuitry is closed via the tissue's impedance, the ABP systemtypically senses a relatively small impedance (under line 150) that maychange, for example, according to changes in the physiologicalproperties of the tissue, for example, while the tissue is treated.Regardless of whether the ABP system's output circuitry is closed oropen, the system/equipment setting, which typically includes cables,flying lead(s), adapter(s), etc., introduces a parasitic impedance thatlowers the total impedance sensed by the ABP system. So, the impedancevariations in system condition 120 (‘closed circuit’)) also reflect thisimpact. As shown in FIG. 1B, the impedance in the ‘closed circuit’ state(120) may vary within ΔZ2, which can be within the range of tenths ofohms to kilo ohms, for example. A low pass filter may be used to removehigh frequency components from the impedance measurements in order toobtain a smoother impedance signal or data during closed circuit state120.

When the treatment device stops touching the treated site, causing theABP system's output circuitry to open (system condition 130), the outputimpedance that the ABP system typically senses in ‘open circuit’ state130 is higher than the impedances usually sensed by the ABP system inthe ‘closed circuit’ state (120), but still, it is much lower thandesired because of the parasitic impedance imposed on the system'soutput circuitry by the periphery equipment setting. A low pass filtermay be used to remove high frequency components in order to obtain asmoother impedance signal or data during open circuit state 130.

The parasitic impedance may, at least at times, be in the same orsimilar order as the tissue impedance, thus making distinguishingbetween the ‘closed circuit’ state and the ‘open circuit’ statedifficult, and sometimes even impossible. As shown in FIG. 1B, theimpedance variations in the ‘open circuit’ state (130) can have amagnitude that may be as large as ΔZ3, which can be within the range oftenths of kilo ohms.

As shown in FIG. 1B, the ‘in-between state’ impedance gap ΔZ4 betweenthe minimum ‘open circuit’ impedance (140) and the maximum ‘closedcircuit’ impedance (150) is much smaller than ΔZ1 (FIG. 1A) (Z1<<ΔZ1;ΔZ14→∞), and it may not enable the ABP system to reliably distinguishbetween the two impedance states. Due to the parasitic impedance causeby the equipment setting, an open circuit impedance may resemble (e.g.,have a value characterizing) a closed circuit impedance, in which casethe ABP system might erroneously determine that the circuit is closed,rather than open, leading to a wrong conclusion that delivery oftherapeutic RF energy should be continued or resumed even though thetherapeutic device no longer touches the treated site. Because impedancegap ΔZ4 is relatively small, it is essential, for proper operation andoptimal results, that the output impedance of the electrosurgicalsystem's output circuitry be accurately and reliably calculated duringthe entire electrosurgical procedure, in order for the electrosurgicalsystem to deliver an optimal electrical energy to the treated organ ortissue whenever it is required (e.g., during closed circuit state).

The present disclosure discloses a method and system for accurately andreliably calculating the output impedance of the electro surgicalsystem's output circuitry. Briefly, the output circuitry of theelectrosurgical system is modelled to a two-port circuit, and a transfermatrix representing the two-port circuit is used in two phases: (1)cable interrogation phase, during which the overall parasiticcapacitance imposed on the electrosurgical system's output circuitry isdetermined, and (2) impedance monitoring phase, during which the valueof the overall parasitic capacitance determined in the first phase isassigned to the transfer matrix, and the transfer matrix is used tocalculate the output impedance of the electrosurgical system's outputcircuitry. (The overall parasitic capacitance imposed on theelectrosurgical system's output circuitry is referred to herein as“leakage capacitance” (Clkg.)

FIG. 2A shows a conceptual two-port network 200 in accordance with thepresent invention. Two-port network 200 may electrically represent anelectrosurgical system. Network 200 may include an electrical circuit(206). Electrical circuit 206 may include circuit elements that mayinclude serial and parallel impedances of an electrosurgical systemrepresented by network 200, or of an output circuitry of theelectrosurgical system. Some of the impedances may represent intrinsic(e.g., internal) impedances of the electrosurgical system. Otherimpedances (e.g., extrinsic impedances) may represent impedances ofperiphery device(s) that may be or include cable(s), connector(s),adapter(s) and an electrosurgical/therapeutic device(s) (e.g., forceps).The periphery devices may be connected to the electrosurgical system,for example during an electrosurgical procedure, and thus be or make theelectrosurgical system's output circuitry. Some of the impedances of, orincluded in, two-port network 200 may represent, or caused by, parasiticcapacitances that undesirably leak electrical current. Network 200 mayrepresent a two-port circuit similar to, for example, an autobipolar(ABP) type electrosurgical system, or it may represent a two-portcircuit that is used by an autobipolar type electrosurgical system todistinguish between various states or conditions of the electrosurgicalsystem, for example the system's open circuit state and closed circuitstate.

A transfer matrix A (202) may be defined, or formed, to representtwo-port network 200. Transfer matrix 202 may be defined or formed, forexample, based on or from the intrinsic impedances and extrinsicimpedances mentioned above. (That is, transfer matrix A may be describedas [A]=f(Zsrc_Int, Zlkg_Int, Zsrc_Ext, Cvirtual), where Zsrc_Int,Zlkg_Int and Zsrc_Ext are the internal and external impedances mentionedabove, and Cvirtual is further discussed below.) Transfer matrix A maybe defined such that it includes a virtual (e.g., software implemented)capacitor, Cvirtual, whose capacitance value may represent the overallparasitic/leakage capacitance value, Clkg, in network 200, or whosevalue may be ‘swept’ to determine the network's parasitic/leakagecapacitance value, Clkg. Once defined or formed, transfer matrix A (202)may be used as shown at formulae (1) and (2), where coefficient All isdefined as A11=Vin/Vout (for Iout=0), coefficient A12 is defined asA12=Vin/Iout (for Vout=0), coefficient A21 is defined as A21=Iin/Vout(for Iout=0), and coefficient A22 is defined as A22=Iin/Iout (forVout=0). (Vin, Iin, Vout and Iout are shown in FIG. 2A.)

Vin=A ₁₁ *Vout+A ₁₂* (Iout)   (1)

Iin=A ₂₁ *Vout+A ₂₂* (Iout)   (2)

or put differently

$\begin{matrix}{\begin{bmatrix}{Vin} \\{Iin}\end{bmatrix} = {\lbrack A\rbrack*\begin{bmatrix}{Vout} \\{Iout}\end{bmatrix}}} & (3)\end{matrix}$

The output impedance (Zout) of network 200 may be calculated usingformula (4).

Zout=Vout/Iout   (4)

Vout and Iout may be found by inverting transfer matrix A andmultiplying the resulting transfer matrix B by Vin and lin, as shown atformula (5):

$\begin{matrix}{\begin{bmatrix}{Vout} \\{Vin}\end{bmatrix} = {\lbrack B\rbrack*\begin{bmatrix}{Vin} \\{Iin}\end{bmatrix}}} & (5)\end{matrix}$

where B=A⁻¹ (A⁻¹ is the inversion of transfer matrix A).

Assuming that transfer matrix B includes, or factors in, the all theparasitic/leakage capacitances, Clkg, existing in two-port network 200;that is, assuming that Cvirtual=Clkg (assuming Clkg is pre-known), thetwo-port network's output voltage (Vout) and output current (Iout) candirectly and easily be calculated for any actual input voltage (Vin) andinput current (Iin), because when Cvirtual=Clkg, transfer matrix A, andtherefore transfer matrix B, genuinely represents the electrical circuitof network 200. To that extent, an actual input voltage (Vin) can beprovided to network 200, and a corresponding input current (lin) may bemeasured in response to voltage Vin. (Iin is a function of Clkg, whichis the actual leakage capacitance in network 200's circuitry.) Then, thenetwork's output impedance (Zout) can be calculated from Vout and Iout.

Vout and Iout can, allegedly, be measured by using a voltmeter and acurrent meter. However, using such meters in electrosurgical environmentwould necessitate additional cabling, and this may be problematic, forexample, because these cables may have to be handled (e.g., movedaround) and they may add leakage capacitance of their own. Using theoutput impedance calculation method disclosed herein renders such metersand cabling unnecessary.

The parasitic/leakage capacitance ‘contribution of the cable (which isincluded in electrosurgical system's output circuitry) to the overallparasitic/leakage capacitance Clkg may be known in advance. For example,it may be read from a barcode or an RFID tag and the read value may beinput/added to transfer matrix B; i.e., assigned to Cvirtual. Since acapacitance per unit of length of a standard cable can usually be foundin a specification sheet, this information and knowing the cable'slength enables determining the contribution of the cable to the overallparasitic capacitance, Clkg. However, there may be cases where thecable's parasitic capacitance is unknown, and even when it is known,‘contribution’ of the other parts of network 200 (e.g., electrosurgicalsystem) may be unknown.

In order to find the value of Clkg, a series of theoretical (e.g.,virtual, software implemented) capacitance values may be assigned/set toCvirtual, for example one capacitance value at a time, and acorresponding output impedance, Zout, may be calculated for eachtheoretical/software capacitance value by using formula (5). Then, amaximum output impedance, Zmax, may be searched for in, or selected orderived from, the group, or series, of the calculated output impedances.The value of the overall parasitic capacitance Clkg resulting in Zmaxmay be set/assigned to the software implemented virtual capacitor,Cvirtual. The way Zmax and Clkg are determined/found is described belowin more detail.

By way of example, the value of Cvirtual may be set to a firstcapacitance value C1 (e.g., C1=50 pF), and a matrix B(1), which is thematrix B corresponding to (factoring in) the first capacitance value(C1), may be calculated with the capacitance value 50 pF factored in.Then, an interrogation input voltage Vin (Vin, FIG. 2A) may be appliedto the circuit, and an input current Iin(1) (Iin, FIG. 2A) may bemeasured for Vin, thus for C1. Then, knowing matrix B(1) and the relatedvoltage-current pair{Vin, Iin(1)}, Vout(1) and Iout(1) may be calculatedby using formula (5). Then, a corresponding output impedance Zout(1) maybe calculated by dividing voltage Vout(1) by current Iout(1) (i.e.,Zout(1)=Vout(1)/Iout(1)). Then, the value of Cvirtual may be set to asecond capacitance value C2 (e.g., C2=60 pF) and a matrix B(2), which isthe matrix B corresponding to C2, may be calculated, this time with thecapacitance value 60 pF factored in. Then, the same interrogation inputvoltage (Vin, FIG. 2A) may be applied to the circuit, and a currentIin(2), which is current Iin measured for C2, may be measured. Then,knowing matrix B(2) and the related voltage-current pair{Vin, Iin(2)},Vout(2) and Iout(2) may be calculated by using formula (5). Then, acorresponding circuit output impedance, Zout(2), may be calculated bydividing voltage Vout by current Iout(2) (i.e.,Zout(2)=Vout(2)/Iout(2)). The same process may be repeated or iteratedas many times as required or desired, to thereby obtain a series ofoutput impedances from which, or based on which, the maximum outputimpedance, Zmax, may be determined. Then, the capacitance, Cvirtual,resulting in, or corresponding to, the maximum output impedance, Zmax,may be determined.

Transfer matrix A (and, therefore matrix B) may be devised such that,during sweeping of the capacitance value of Cvirtual, for example in therange of 50 pF-600 pF, the maximum output impedance, Zmax, occurs whenthe value of Cvirtual matches that of the overall parasitic capacitance,Clkg, of network 200; i.e., when Cvirtual=Clkg. Cvirtual=Clkg occurswhen Zout=Zmax because the system's model, as embodied in the transfermatrix, is closest to being correct when Zout has the highest value.Every output impedance, Zout, is calculated for an open circuitcondition in order for the measured input current, Iin, which is anactual electrical current, to reflect or represent the electricalcurrent leaked by the leakage capacitance, without having any electricalload affecting this current. The higher the output impedance, Zout,calculated for any particular virtual impedance (−1/jwC_(virtual))inserted into the model (e.g., transfer matrix), the closer theparticular virtual impedance is to the actual cable's impedance, and,therefore, the closer the value of the virtual capacitance is to that ofthe leakage capacitance.

FIG. 2B schematically illustrates a two-port network 204 according to anexample embodiment. Two-port network 204 may electrically represent(e.g., model) an electrosurgical system. Complex impedances Zsrc_Int andZlkg_Int may represent intrinsic parasitic (leakage) complex impedancesinside an electrosurgical system (e.g., system 400 of FIG. 4). Compleximpedances Zsrc_Ext and Zlkg_Ext may represent extrinsic parasiticcomplex impedances of, caused by or associated with, a periphery system,which is referred to herein as the “output circuitry”, that may beconnected to the electrosurgical system (e.g., output circuitry 420).(The term ‘output circuitry’ may refer to or include any device,apparatus or system that is, or can be, electrically connected to (e.g.,plugged into) an electrosurgical system, via/through which theelectrosurgical system delivers, or may deliver, ‘interrogation’ signalsduring an impedance monitoring phase, or therapeutic energy to a treatedsite during electrosurgical procedure.) Two-port network 204 may beanalyzed as two sub networks: sub network 210 and sub network 220. Eachsub network may likewise be analyzed, for example as described below. Inconnection with FIG. 2B, unless specifically stated otherwise, an outputvoltage and an output current are voltage and current at the output ofthe discussed sub network. For example, a voltage that may be regardedas input voltage of/for sub network 220, is an output voltage for subnetwork 210.

Impedance Zload may represent an impedance of a treated site (e.g.,tissue) during electrosurgical procedure. Impedance Zlkg_Ext mayrepresent an overall parasitic (leakage) capacitance, Clkg, of network204 or sub network 220, which leaks current I3. Ideally, Iin=0 whenClkg=0 pF and the electrosurgical device does not touch a tissue.However, since the impedance due to the leakage capacitance (i.e.,Xc=(−1/jwC)) might, at times, resemble impedance Zload (the impedance ofthe tissue), for example it may have a value in the same order asimpedance Zload, there is some non-zero current (I3>0Amp) that leaks viathe leakage capacitance, and since Iin=I2+I3=I3, knowing only the valueof current Iin may not suffice to decisively determine the correct valueof the system's output impedance and, therefore, and it may not sufficeto distinguish between the ‘open circuit’ state and the ‘closed circuit’state of network 204 or network 220.

Assuming output voltage V2 and output current I2 of a two-port networkare known, an input voltage V1 and an input current I1 may be calculatedusing equations (6) and (7), where A11 and A22 are dimensionlesscoefficients, A12 is impedance and A21 is admittance. The fourcoefficients may be calculated using the definitions specified above inconnection with formulae (1) and (2).

V1=A ₁₁ V2+A ₁₂(−I2)   (6)

I1=A ₂₁ V2+A ₂₂(−I2)   (7)

A total impedance, Ztotal (also referred to as Zout 280), may becalculated using formula (8):

$\begin{matrix}{{Ztotal} = {{Zsrc} + \frac{{Zlkg} \cdot {ZLoad}}{{Zlkg} + {ZLoad}}}} & (8)\end{matrix}$

Voltage Vin at the input of circuit 220 may be calculated using formula(9):

$\begin{matrix}{{Vin} = \frac{{Iin} \cdot \left( {{{Zlkg} \cdot {Zsrc}} + {{Zlkg} \cdot {ZLoad}} + {{Zsrc} \cdot {ZLoad}}} \right)}{{Zlkg} + {ZLoad}}} & (9)\end{matrix}$

Accordingly, current I1 (I1=Iin) may be calculated using formula (10):

$\begin{matrix}{{I\; 1} = \frac{{Vin} \cdot \left( {{Zlkg} + {ZLoad}} \right)}{{{Zlkg} \cdot {Zsrc}} + {{Zlkg} \cdot {ZLoad}} + {{Zsrc} \cdot {ZLoad}}}} & (10)\end{matrix}$

The output voltage (Vout) on Zload (Vout=V2) may be calculated usingformula (11) below, and the current (Iout) through Zload (Iout=I2) maybe calculated using formula (12) below.

$\begin{matrix}{{Vout} = \frac{{Vin} \cdot {Zlkg} \cdot {ZLoad}}{{{Zlkg} \cdot {Zsrc}} + {{Zlkg} \cdot {ZLoad}} + {{Zsrc} \cdot {ZLoad}}}} & (11) \\{{Iout} = \frac{{Vin} \cdot {Zlkg}}{{{Zlkg} \cdot {Zsrc}} + {{Zlkg} \cdot {ZLoad}} + {{Zsrc} \cdot {ZLoad}}}} & (12)\end{matrix}$

Using the equations and the coefficient definitions above, and assumingthat Zload is infinite (because the circuit analysis performed during acable interrogation phase, which is described in more detail below, isperformed when the electrosurgical system is in an open circuit state),the resulting matrix coefficients A₁₁, A₁₂, A₂₁ and A₂₂ are:

$\begin{matrix}{{A\; 11} = {\frac{Zsrc}{Zlkg} + 1}} & (13) \\{{A\; 12} = {Zsrc}} & (14) \\{{A\; 21} = \frac{1}{Zlkg}} & (15) \\{{A\; 22} = 1} & (16)\end{matrix}$

A corresponding transfer matrix Aint would, therefore, be as shown below(17).

$\begin{matrix}{{Aint} = \begin{pmatrix}{\frac{Zsrc}{Zlkg} + 1} & {Zsrc} \\\frac{1}{Zlkg} & 1\end{pmatrix}} & (17)\end{matrix}$

Coefficients A₁₁ and A₂₁ of matrix Aint are a function of the value of acorresponding leakage impedance Zlkg (‘int’ in ‘Aint’ means ‘internal’,which means “in the electrosurgical system itself”.) This enablescalculating the circuit's output voltage and output current with theeffect of a parasitic capacitance factored in.

According to transmission matrix theory, A*[out]=[in]. (Inputparameters, [in], are equal to output parameters, [out], multiplied bymatrix A.) To calculate the circuit output [out], matrix Aint isinverted, Aint⁻¹=B, so that B*[in]=[out]. This enables calculating theoutput voltage and the output current of the circuit from the inputvoltage and input current, assuming that transfer matrix B is known.

Inverting matrix Aint above results in matrix Bint (Bint=Aint⁻¹) (18)below.

$\begin{matrix}{{Bint} = \begin{pmatrix}1 & {- {Zsrc}} \\{- \frac{1}{Zlkg}} & \frac{{Zlkg} + {Zsrc}}{Zlkg}\end{pmatrix}} & (18)\end{matrix}$

The output voltage, Vout, and the output current, Iout, at the load arefunction of the input voltage (Vin) and the input current (Iin)multiplied by matrix B, as shown in formula (19).

B matrix*V/I in=V/I out   (19)

Therefore, the output voltage, Vout, and the output current, Iout, canbe calculated by multiplying matrix B by Vin and Iin, as shown informula (20).

$\begin{matrix}{{Out}\mspace{14mu} \text{:=}\mspace{14mu} {\begin{pmatrix}1 & {- {Zsrc}} \\{- \frac{1}{Zlkg}} & \frac{{Zlkg} + {Zsrc}}{Zlkg}\end{pmatrix} \cdot \begin{pmatrix}{Vi} \\{Ii}\end{pmatrix}}} & (20)\end{matrix}$

In order to find (and later use) a single matrix B (Ball) for the entirecircuit (e.g., for entire network 204) that will combine (factor in) theimpedance (e.g., parasitic capacitance) effect of both the sub network210 and sub network 220, a matrix Bint corresponding to circuit 210 isto be multiplied by a matrix Bext corresponding to circuit 220; i.e., anoverall matrix, Ball, may be found such that Ball=Bext*Bint. To do that,the matrices Bext and Bint have to be found. Matrix Bint is the same asmatrix B in equation (20) above except that ‘B’ changes to ‘Bint’,‘Zsrc’ changes to ‘ZsrcInt’ and ‘Zlkg’ changes to ‘ZlkgInt’, as shown atformula (21) below.

$\begin{matrix}{{Bint}\mspace{14mu} \text{:=}\mspace{14mu} \begin{pmatrix}1 & {- {ZsrcInt}} \\{- \frac{1}{ZlkgInt}} & \frac{{ZlkgInt} + {ZsrcInt}}{ZlkgInt}\end{pmatrix}} & (21)\end{matrix}$

The internal source impedance (‘ZsrcInt’) and internal leakage impedance(‘ZlkgInt’) of matrix Bint, which are referred to as “internal cablecompensation”, may be determined during calibration of theelectrosurgical system, and since these values do not changesignificantly during electrosurgical procedure and from oneelectrosurgical procedure to another, they can be hardcoded into theelectrosurgical system's software. (The electrical parameters of theinternal cable compensation typically depend on the cable layout and theprinted circuit board (PCB) layout internal to the electrosurgicalsystem, and they are determined per type of electrosurgical generator.)

Matrix Bext is the same as matrix B in equation (20) above except that‘Bint’ changes to ‘Bext’, ‘Zsrc’ is changed to ‘ZsrcExt’ and ‘Zlkg’ ischanged to ‘ZlkgExt’, as shown at formula (22) below.

$\begin{matrix}{{Bext}\mspace{14mu} \text{:=}\mspace{14mu} \begin{pmatrix}1 & {- {ZsrcExt}} \\{- \frac{1}{ZlkgExt}} & \frac{{ZlkgExt} + {ZsrcExt}}{ZlkgExt}\end{pmatrix}} & (22)\end{matrix}$

Accordingly, the overall matrix B (Ball) corresponding to circuit 204 isas shown below (formula 23):

$\begin{matrix}{{B = {{B_{ext} \cdot B_{int}} = {\begin{bmatrix}B_{11} & B_{12} \\B_{21} & B_{22}\end{bmatrix}{where}}}},{B_{11} = {{\frac{Z_{srcExt}}{Z_{lkgInt}} + {1B_{12}}} = {{{- \left( {Z_{srcInt} + \frac{Z_{srcExt}\left( {Z_{lkgInt} + Z_{srcInt}} \right.}{Z_{lkgInt}}} \right)}B_{21}} = {- \left( {{\frac{1}{Z_{lkgExt}} + {\frac{Z_{lkgExt} + Z_{srcExt}}{Z_{lkgInt}Z_{lkgExt}}{and}}},{B_{22} = {\frac{Z_{srcInt}}{Z_{lkgExt}} + \frac{\left( {Z_{lkgInt} + Z_{srcInt}} \right)\left( {Z_{lkgExt} + Z_{srcExt}} \right)}{Z_{lkgInt}Z_{lkgExt}}}}} \right.}}}}} & (23)\end{matrix}$

As shown above, matrix B (formula (23) above) includes four variables:(1) Zsrclnt, (2) Zlkglnt, (3) ZsrcExt, and (4) ZlkgExt. (Zload in MatrixB is infinite in order to represent an ‘open circuit’ state.) VariablesZsrclnt, Zlkglnt and ZsrcExt of matrix B are relatively constant (andknown) per used electrosurgical system and periphery equipment. VariableZlkgExt represents a virtual capacitor (e.g., Cvirtual 250, FIG. 2B)that is added to the transfer matrix in order to represent the leakagecapacitance of an output circuitry (e.g., output circuitry 220) that is,or may be, connected to the electrosurgical system.

If the value of the (total) leakage capacitance of electrosurgicalsystem 204, or of output circuitry 220, is known (whether in advance, orafter manipulation of the value of Cvirtual), it may be added to atransfer matrix such as transfer matrix B. That is, the known leakagecapacitance value may be assigned or set to the virtual capacitor,Cvirtual (shown at 250), and the transfer matrix, with the leakagecapacitance value assigned/set to Cvirtual 250, may be used, for exampleduring an impedance monitoring phase, to calculate an output electricalparameter of two-port network 204, or two-port network 220, such asoutput voltage Vout, output current Iout, output impedance (280) andoutput power (Pout) that is delivered to a treated site (e.g., bodilyorgan, bodily tissue, etc.). However, if the parasitic capacitance ofelectrosurgical system 204, or of output circuitry 220, is unknown, itmay be determined/found by sweeping, during a cable interrogation phase,the value of Cvirtual, and, for each value of Cvirtual, applying avoltage V1 (or Vin) and measuring an input current I1 (or Iin) todetermine, or calculate, an optimal capacitance value for Cvirtual thatrepresents, is equal to, or resembles the actual value of the parasiticcapacitance. The method and process used to determine the optimalcapacitance value for the virtual capacitor, Cvirtual, is described inmore detail below.

FIG. 3 shows an example impedance-capacitance plot 300 in accordancewith an example embodiment. The horizontal (X) axis of plot 300indicates virtual capacitance values assigned/set to a virtualcapacitor, Cvirtual, such as virtual capacitance 250 of FIG. 2B. Thevertical (Y) axis indicates respective values of an output impedancesimilar to Zout 280 of FIG. 2B when the external circuit (e.g., externalcircuit 220 of FIG. 2B) is open; that is, when the electrosurgicalsystem is in ‘open circuit’ state. (Sweeping the value of Cvirtual is tobe done in the system's open circuit state in order to ensure that, forany given input voltage (Vin), the input current resulting from Vinreflects (is a function of) the leakage capacitance Clkg, or Zlkg_Ext,and is not affected by extraneous impedance (e.g., Zload; e.g., tissue'simpedance, etc.).

A value of a virtual capacitor may be changed (swept) across a series ofcapacitance values, and an output impedance 280 may be calculated foreach capacitance value from the pertinent output voltage and outputcurrent, which, in turn, are calculated by multiplying the input voltageand current by a transfer matrix, which is updated with the relevantcapacitance value (Equation (19), for example, may be used to calculatean output voltage and current for each value of the virtual capacitance,Cvirtual.) Referring to example plot 310, Clkg=5 pF may result in anoutput impedance of approximately 2KΩ, Clkg=135 pF may result in anoutput impedance of approximately 5KΩ, Clkg=200 pF may result in anoutput impedance of approximately 20KΩ, Clkg=215 pF may result in anoutput impedance of approximately 35KΩ, etc.

Curve 310 may be fitted to the impedance values, and a maximum outputimpedance, Zmax, may be derived from curve 310 using any suitablemathematical or digital signal processing (“DSP”) method. By way ofexample, in FIG. 3 Zmax is approximately 35KΩ, and the capacitance value(an ‘optimal capacitance value’) of the virtual capacitor resulting inZmax is approximately 215 pF. Zmax is shown as an apex 320. Apex point320 is very conspicuous, which makes it very discernible, hence easy todetect/identify.

Sweeping of the virtual capacitor's value is to be performed when theexternal circuit (e.g., external circuit 220 of FIG. 2B is open (i.e.,when Zload is disconnected). Therefore, curve 310 represents variousopen circuit conditions of (i.e., various current leakages in) theexternal circuit. In order to ensure that a transfer matrix genuinelyrepresents network 204 (FIG. 2B) during an impedance monitoring phase,the capacitance value of the virtual capacitor (e.g., 215 pF in theexample plot 310) in the transfer matrix, which results in the maximumoutput impedance (e.g., 35KΩ, per plot 310) may be assigned/set, as anoptimal capacitance value to Cvirtual during the impedance monitoringphase.

The electrical properties (e.g., impedances) characterizing circuit 204of FIG. 2B may vary from one electrosurgical system to another and fromone electrosurgical system's peripheral to another (e.g., depending onthe type of the used cable(s) and/or type of therapeutic device, etc.).The electrical properties characterizing circuit 204 may change evenduring electrosurgical treatment, for example, due to manipulation ofthe system's cable(s) and therapeutic device by the surgeon. Changes inthe electrical properties of circuit 204 may change the shape of theresulting Z-C curve. For example, a change in the electrical propertiesof circuit 200 may cause the curve to be lower (i.e., have lowerimpedance values) or higher (i.e., have higher impedance values), or itmay cause curve apex 320 to move in one direction or another and,consequently, change the optimal capacitance value that is used duringimpedance monitoring phase. However, whatever the resulting Z-C curveis, its apex may remain relatively high and conspicuous. (The smaller adamping resistance in circuit 204 of FIG. 2B (i.e., the less lossy thecircuit), the higher the circuit's Q factor and, consequently, thenarrower the spike width (e.g., spike width 330), which makes the curveapex conspicuous.) Since the impedance of a tissue is mostly resistive,it has only little effect, if any, on the location of the curve's apexon the capacitance axis, hence on the value of the virtual capacitanceresulting in the maximum output impedance (Zmax).

By repeating the cable interrogation process described herein, say, oncein a while (e.g., once every few minutes), or based on a predefinedcriteria (e.g., an instantaneous output impedance sensed exceeding athreshold value; e.g., 10 KΩ) a new graph/plot may be calculated orfitted to accommodate for changes in the electrical properties of/in anypart of circuit 204 of FIG. 2B and from which corresponding virtualcapacitance may be selected. For example, when a cable connected to anelectrosurgical system is replaced with a different type or model ofcable and/or when the type of therapeutic device connected to the cableis replaced with a different type of therapeutic device, the elements ofthe transfer matrix B (see, e.g., formula (23) above) may be updatedaccording to the electrical parameters of the new cable and therapeuticdevice (e.g., using relevant information in their manuals orspecification sheet, etc.). Then, a new cable interrogation process maybe initiated, by using the updated transfer matrix, to find a newcapacitance value that is suitable for use with the new cable and/ortherapeutic device. Then, the transfer matrix with/including the newcapacitance value may be used to monitor an electrosurgical system'soutput electrical parameter (e.g., output voltage , output current,output impedance, output electrical power), during an impedanceinterrogation phase, as described herein.

FIG. 4 is a block diagram of a system 400 for treating a bodily organ ortissue according to an example embodiment. System 400 may include anelectrosurgical system 410 and an output circuitry 420 connectable toelectrosurgical system 410.

Electrosurgical unit 410 may include a signal generator 430 (e.g., RFsignal generator) for generating, among other things, low energyimpedance interrogation signals, for example less than 1 watt, forexample at a frequency of 80 kHz, for example, during an impedancemonitoring/interrogation phase. Signal generator 430 may also generatesimilar low energy signals during a cable interrogation phase. Signalgenerator 430 may also generate high energy (e.g., 1-70 watt) RFtherapeutic signal that electrosurgical system 410 may deliver, duringan electrosurgical procedure, to a bodily organ or tissue (e.g.,electrosurgical site 404) of a subject 406 via, through or using outputcircuitry 420. Electrosurgical system 410 may also include a voltagemonitoring circuit (“VMC”) 440, a current monitoring circuit (“CMC”)450, an impedance-capacitance (Z-C) table 460 and a memory 470.

Output circuitry 420 of electrosurgical system 410 may include atherapeutic electrosurgical device 402 via/by which electrosurgicalsystem 410 may deliver RF therapeutic energy from RF signal generator430 to bodily organ, or tissue, 404 of subject 406 in, or during, aclosed circuit state (of electrosurgical system 410 or output circuitry420) in which therapeutic device 402 closes an output ‘port’ 408 ofoutput circuitry 420 via bodily organ/tissue 404. Output circuitry 420may also include a cable system for connecting therapeutic device 402 toelectrosurgical system 410. The cable system may include at least acable 412, a connector or adapter 414 via which cable 412 may beconnected to electrosurgical system 410, and a connector or adapter 416via which cable 412 may be connected to electrosurgical device 402.Electrosurgical device 402 may be or include a bipolar instrument/toolincluding two electrode tines (418, 422) for performing various surgicaloperations, for example coagulation, ablation, cutting and/or otheroperations.

Electrosurgical device 402 may, at times (e.g., occasionally,intentionally, unintentionally), touch bodily organ or tissue 404, inwhich case electrosurgical system 410 may sense (at 424), throughelectrode tines 418 and 422 (that make up, or form, output port 408 ofoutput circuit 420), an output impedance, Zout, which is, or represents,the tissue's impedance (426) or approximately the tissue's impedance,which may be relatively low (e.g., a few ohms to a few kilo ohms). Asystem state where electrosurgical device 402 touches bodily organ ortissue 404, which results in a relatively low output impedance, Zout, asmay be sensed, for example, by electrosurgical system 410 at 424, isreferred to herein as a ‘closed circuit’ state of electrosurgical system410 or output circuitry 420 of electrosurgical system 410.

Electrosurgical device 402 may, at other times, not touch bodily organor tissue 404, in which case electrosurgical system 410 is to,theoretically, sense (at 424) an output impedance, Zout, that isinfinitely high (428). A system state where electrosurgical device 402does not touch bodily organ or tissue 404, which should theoreticallyresult in a relatively very high output impedance, Zout, as may besensed, for example, by electrosurgical system 410 at 424, is referredto herein as an ‘open circuit’ state of electrosurgical system 410 oroutput circuitry 420 of electrosurgical system 410. However, inpractice, due to parasitic (leakage) capacitances due to or caused by,for example, cable 412, connectors/adapters 414 and 416 and/or byelectrosurgical device 402 and/or by additional circuit elements (e.g.,gripping the electrosurgical device by a surgeon), the output impedance,Zout, that electrosurgical system 410 may sense at 424 is, or can be, inthe order of the tissue's impedance.

Controller 480 may receive (e.g., from a user; e.g., surgeon, operatingthe system) an input signal or message 482 instructing controller 480 tooperate electrosurgical system 410 in a cable interrogation mode duringa cable interrogation phase, or in an impedance interrogation modeduring an impedance monitoring phase, or in RF therapeutic energydelivery mode during therapeutic RF energy delivery phase. (Controller480 may, in the RF therapeutic energy delivery mode, cause RF generator430 to generate RF therapeutic energy, and it may cause electrosurgicalsystem 410 to deliver the RF therapeutic energy to organ/tissue 404.)Alternatively or additionally, controller 480 may determine when eachoperation mode should be activated, deactivated, resumed, etc. based onsignal or message it may receive from either VMC 440 or CMC 450, or fromboth monitoring circuits.

After cable 412 and electrosurgical device 402 are connected to system410), controller 480 may perform a cable interrogation process in orderto find an optimal capacitance value that is suitable for (e.g.,represents, or is equal or similar to the leakage capacitances causedby) the particular cable and therapeutic device 402.

Controller 480 may, during or for the cable interrogation phase, definea transfer matrix for output circuit 420 when output circuitry 420 isopen (e.g., when therapeutic device 402 is detached from organ/tissue404), and store the transfer matrix, for example, in memory 470. Thetransfer matrix may include a virtual capacitor, and controller 480 maychange the value of the virtual capacitor in order to change the outputimpedance, Zout, of output circuitry 420 in order to identify a maximumoutput impedance (Zmax). That is, during the cable interrogationprocess, controller 480 may (virtually) sweep the value of the virtualcapacitor, Cvirtual, and, for each selected value of Cvirtual,controller 480 may calculate an output impedance, Zout, for that valueby using the transfer matrix. Controller 480 may, then, update thematrix information, which may be stored, for example, in memory 470,with the value of the virtual capacitance.

During the cable interrogation phase, controller 480 may store, forexample in Z-C table 460, capacitance values it selects for the virtualcapacitance and also output impedances that controller 480 respectivelycalculates for these capacitance values. After controller 480selects/uses up, and stores, all the virtual capacitance values to beselected/used, controller 480 may identify an output impedance havingthe greatest value as the maximum output impedance, Zmax, and determinethe associated (i.e., related optimal) virtual capacitance thatcontroller 480 may use during the impedance interrogation phase.(Controller 480 may select Zmax from the stored output impedances, or itmay interpolate it from the stored output impedances.)

Controller 480 may be configured to, during the impedance interrogationphase, monitor an instantaneous output electrical parameter of outputcircuitry 420 (e.g., voltage, current, impedance, electrical powerdelivered to the treated site) by/while using the optimal capacitancevalue. (The ‘optimal capacitance value’ is the capacitance valueassigned to the virtual capacitance, which result in the maximum outputimpedance, Zmax.) Using the optimal capacitance value, which may changefrom one cable interrogation to another, enables controller 480 todetermine, for example, the instantaneous output impedance and/or outputelectrical power of the output circuitry accurately and reliably. Usingthe optimal capacitance value also enhances the controller 480 abilityto distinguish between the closed circuit state (in which controller 480switches (490) signal generator 430 on) and the open circuit state ofelectrosurgical system 410, or output circuitry 420 (in which controller480 switches (490) signal generator 430 off). Controller 480 may alsouse the output electrical power to control (490) signal generator 430,for example, such that the therapeutic energy delivered to the treatedsite during therapy is optimal throughout the therapeutic procedure.

During the impedance interrogation phase, controller 480 may calculatethe system's instantaneous output impedance, Zout, resulting, forexample, from the cable (e.g., 412), cable connectors (e.g., 414, 416),cable adapters, therapeutic device (402), and bodily organ/tissue 404(if therapeutic device 402 touches bodily organ/tissue 404) byoutputting, or applying, an interrogation voltage, Vin, betweenterminals 432 and 434 (and measuring that voltage by VMC 440), andconcurrently measuring the resulting interrogation current Iin (452) byCMC 450. Then, controller 480 may multiply voltage Vin and the measuredcurrent lin by the transfer matrix, in which the optimal capacitancevalue is set to the virtual capacitor, to obtain a corresponding outputvoltage (Vout) and output current (Iout) at the electrosurgical device'selectrode tines 408. Controller 480 may calculate the output impedanceZout (Zout=Vo/Io) and, based on its value, determine the next operationmode of electrosurgical system 410. ‘Knowing’ the output voltage (Vout)and output current (Iout) at the electrode tines 408, controller 480 mayalso determine the therapeutic energy actually delivered to the treatedsite. Knowing the actual therapeutic energy delivered to the treatedsite, controller 480 may control (490) the operation of signal generator430 (e.g., it may control an electrical parameter of the generator) todeliver an optimal amount of therapeutic energy to the treated site atany given time during the RF therapeutic delivery phase.

Signal generator 430 may controllably deliver therapeutic RF energy to atreated site. When signal generator 430 is not delivering therapeutic RFenergy, it may deliver, at times, a relatively small averageinterrogation current 452 (e.g., in the order of micro amps), forexample in compliance with IEC safety regulations, to interrogate orsense the presence and magnitude of a tissue impedance.

FIG. 5 shows a method of operating an electrosurgical system (e.g.,system 400 or system 410, FIG. 4) according to an example embodiment.Assume that the electrosurgical system is connected to, or include, asignal generator (e.g., signal generator 430, FIG. 4) and an outputcircuitry (e.g., output circuitry 420, FIG. 4) including an electricalcable (e.g., cable 412, FIG. 4) connecting an electrosurgical device(e.g., signal generator 402, FIG. 4) to the electrosurgical system. Atstep 510, apply or provide an input voltage (Vin) to the electricalcable, and concurrently measure an input current (Iin) in the electricalcable in response to Vin.

At step 520, calculate an electrical parameter of the output circuitryfrom the input voltage (Vin) and the input current (Iin) and by using atransfer matrix electrically representing the output circuitry. Theelectrical parameter of the output circuitry may be selected from thegroup consisting of output voltage, output current, output impedance andoutput electrical power that is delivered to the treated site. In casethe electrical parameter is an output impedance of the output circuitry,the method may include determining the operation mode of or for theelectrosurgical system based on a value of the output impedance, and/ordistinguishing, based on the value of the output impedance, between an‘open circuit’ state of the output circuitry in which theelectrosurgical device is detached, or removed, from a treated site, anda ‘closed circuit’ state of the output circuitry in which theelectrosurgical device touches the treated site. The value of the outputimpedance of the output circuitry may be determined in the way describedherein.

In case the electrical parameter is an output electrical power deliveredvia the output circuitry to the treated site, the method may includecalculating an output voltage (Vout) and an output current (Iin) of theoutput circuitry by multiplying Vin and Iin by the transfer matrix. Themethod may further include controlling the output electrical power toprovide an optimal therapeutic energy to the treated site.

At step 530, determine an operation mode of or for the electrosurgicalsystem based on the value of the electrical parameter. The operationmode may be selected from the group consisting of: (i) delivering, by asignal generator (e.g., signal generator 430, FIG. 4), via theelectrosurgical device, therapeutic energy to a treated site, (ii)adjusting an electrical parameter of the signal generator when thetherapeutic energy is delivered to the treated site, (iii) refrainingfrom delivering therapeutic energy to the treated site, (iv) cableinterrogation mode and (v) impedance monitoring mode.

FIG. 6 shows a method of operating an electrosurgical system accordingto an example embodiment. In general, the method includes stepsimplementing, or used during, a cable interrogation phase during which acapacitance value representing leakage capacitance in theelectrosurgical system may be selected for use in a transfer matrix thatrepresents the electrosurgical system, or part thereof, including thesystem's parasitic capacitance. The method may also include stepsimplementing, or used during, an impedance monitoring phase during whichthe electrosurgical system may use the selected capacitance valuerepresenting the leakage capacitance in the electrosurgical system tomonitor the instantaneous an output impedance of the electrosurgicalsystem. Steps 610 and 620 (inclusive) are directed to the cableinterrogation phase. Steps 630 through 660 are directed to the impedanceinterrogation/monitoring phase.

Assume that the electrosurgical system includes, or is connected to, anoutput circuitry comprising an electrical cable connected to anelectrosurgical device. At step 610, defining a transfer matrix, thetransfer matrix electrically representing the output circuitry andcomprising a virtual capacitor (Cvirtual) to represent a leakagecapacitance (Clkg) in the output circuitry, and at step 620, assign tothe virtual capacitor (Cvirtual) a capacitance value representing theleakage capacitance (Clkg). The way the capacitance value to be assignedor set to the virtual capacitor (Cvirtual) is selected is describedherein. For example, the capacitance value may be obtained automatically(e.g., by reading it from an external device associated with the cable;e.g., from an RFID tag or a barcode, or through sweeping of thecapacitance value of the virtual capacitance, as described herein) ormanually.

Steps 630 through 660 may be performed during an electrosurgicalprocedure (e.g., during an impedance monitoring phase). At step 630,apply or provide, by the electrosurgical system, an input voltage (Vin)to the electrical cable and measure an input current (Iin) in theelectrical cable. At step 640, calculate an output impedance (Zout) ofthe output circuitry from the input voltage (Vin) and the input current(Iin), and by using the transfer matrix, and, at step 650, compare theoutput impedance (Zout) to an impedance threshold value. Comparing theoutput impedance of the system to a threshold value may includedetermining, based on the impedance comparison result, whether theoutput circuitry is in an ‘open circuit’ state in which theelectrosurgical device does not touch a bodily organ or tissue, or in a‘closed circuit’ state in which the electrosurgical device touches thebodily organ or tissue.

At step 660, determine an operation mode of the electrosurgical systembased on the comparison result. Determining an operation mode of theelectrosurgical system may include, for example, determining whether theelectrosurgical device is connected to the electrical cable, or whetherthe electrosurgical device touches a treated site. (Other or additionaloperation modes may be determined or use, as exemplified herein.)

The output impedance (Zout) of the electrical cable may be found in theway described herein, for example it may be found by calculating, byusing the transfer matrix, an output voltage (Vout) and an outputcurrent (Iout) for the output circuitry from the input voltage (Vin) andfrom the input current (lin), etc. The output voltage (Vout) and outputcurrent (Iout) may additionally or alternatively be used to calculate anelectrical power (Pout) that is delivered to a treated site from theelectrosurgical system via the electrosurgical device.

FIG. 7 shows a block diagram demonstrating usage of the methodsdisclosed herein by an electrosurgical system according to an exampleembodiment. Initially, a cable and therapeutic device are connected toan electrosurgical system, and default cable compensation parameters maybe used. (The term ‘cable compensation parameters’ refer to a transfermatrix representing the electrosurgical system, or part thereof; e.g.,an output circuitry connected to the electrosurgical system.)

At block or condition 710 (“Startup”) the electrosurgical system checkswhether a cable is connected to it, and, if a cable is connected to it,whether the connection is intact. Since at this stage the generatordelivering therapeutic energy is shut down (switched off), it isexpected that the output impedance, Zout, which is calculated by theelectrosurgical system during the cable interrogation phase disclosedherein, should be greater than a first predetermined impedance thresholdvalue (Zmin), for example Zout should be greater than, say, Zmin=800ohms. (Other threshold values may be used.) If Zout is lower than thethreshold value (Zout<Zmin), the electrosurgical system may wait apredetermined time period (712), and if Zout, which the electrosurgicalsystem repeatedly calculates, remains lower than Zmin for a period thatis longer than the predetermined time period, the electrosurgical systemmay indicate this condition (‘Zout too low’) to the operator and abortoperation, because this condition may indicate, for example, a faultycable. However, if, Zout>Zmin during the predetermined time period, theelectrosurgical system may assume that a cable is connected to theelectrosurgical system and, accordingly, cancel the ‘Zout too low’indication, and it may continue to block or condition 720 (“VerifyCable”).

At block or condition 720 the electrosurgical system may check, forexample based on Zout, whether the electrosurgical system is in ‘opencircuit’ state, that is, whether the cable (and possibly theelectrosurgical device) are ready for use. If Zout is lower than asecond impedance threshold value Zopen (e.g., Zopen=10KΩ. Otherthreshold values may be used), it may be assumed that there may be aproblem with the cable, or electrosurgical device, or both. Therefore,the electrosurgical system may wait a predetermined time period (at722), and if Zout<Zopen for more than the specified time period, theelectrosurgical system may indicate this condition (‘Cable notdetected’), for example to the operator, and abort operation, because itmay be that no cable is connected to the electrosurgical system, or thatthe cable was disconnected, for example unintentionally, or that not allcable wires are properly connected. However, if, Zout>Zopen, theelectrosurgical system may cancel the ‘Cable not detected’ indication,and continue to block 730 (“Cable Interrogation”) to initiate a cableinterrogation phase.

At block or condition 730 the electrosurgical system continues tomonitor the system's output impedance in order to determine whethertherapeutic energy can be delivered to a treatment site. Theelectrosurgical system may use a third impedance threshold value, Zstart(e.g., Zstart=2.2KΩ. Other threshold values may be used; e.g., 2.5 KΩ,by the electrosurgical system to determine when it can delivertherapeutic energy to the treated site. If the condition Zout<Zstart ismet, the electrosurgical system may move on (732) to block or conditionblock 740 (“Wait for Start”), where a delay timer may be activated. Ifthe condition Zout<Zstart is met uninterruptedly for a period of time(which may be, for example, between 0 seconds and 2.5 seconds) that maybe set by block 740, this means that the electrosurgical device has nottouched the treated site by accident but, rather, the system operator isready to start, or resume, therapy. Therefore, the (current) cableinterrogation session can be terminated, at least temporarily, andtherapeutic energy can be generated and delivered (744) through theelectrosurgical device. Otherwise (i.e., if Zout is not lower thanZstart for the specified time period), cable interrogation condition maybe resumed (742).

At block or condition 750 (“Active Stage”) the electrosurgical systemdelivers therapeutic energy to the treated site. Block 750 may be exitedin one of three ways: (1) the operator of the electrosurgical system maydecide to abort (770) the electrosurgical procedure, or (2) after atimeout is indicated (752), or (3) when Zout is greater than a thirdimpedance threshold value, Zstop (754) (e.g., Zstop=3.5KΩ). (Otherthreshold values may be used.) If timeout is reached, this means thatthe electrosurgical device should be removed from the treated site inorder not to overheat the treated tissue/organ. If Zout>Zstop, thismeans that the electrosurgical system should stop delivering therapeuticenergy delivery, and commence a new cable interrogation session. In bothcases (2) and (3) the electrosurgical system waits, in block 760 (“Waitfor Open Circuit”), for the operator to move the electrosurgical deviceaway from the treated site, that is, it continues to monitor the outputimpedance and waits until Zout>Zopen. Then, electrosurgical system mayrevisit block 730 and repeat the process as described above.

FIG. 8 schematically illustrates utilization of methods according to anexample embodiment. Various events occurring in an electrosurgicalsystem are chronologically shown with respect to timeline 810. Graph 820illustrates a status of a signal generator (e.g., signal generator 430,FIG. 4) generating, at times, high energy therapeutic signal, and, atother time, lower energy therapeutic signal.

With reference to timeline 810, an electrosurgical system's operationcycle 812 may include a cable interrogation period 814, to obtain acapacitance value representing leakage capacity of an electrosurgicalsystem, or part thereof, and a period 816 during which theelectrosurgical system's instantaneous output impedance is monitored(interrogated) by using the result of the cable interrogation result;i.e., by using a transfer matrix that electrically represents theelectrosurgical system, or part thereof, and includes the capacitancevalue representing the leakage capacitance in, or of, theelectrosurgical system, or part thereof.

During cable interrogation period 814, a capacitance value C1 (e.g.,C1=50 pF) is assigned, at time t1, to a virtual capacitor in thetransfer matrix, and a corresponding output impedance Zout(1) iscalculated for C1. At time t2, a capacitance value C2 (e.g., C1=60 pF)is assigned to the virtual capacitor, and a corresponding outputimpedance Zout(2) is calculated for C2, and so on (for C3 through C61 attime t61). A maximum output impedance, Zmax, is derived from the seriesof impedances Zout(1)-Zout(61), and an optimal capacitance value, whichis a capacitance value resulting in Zmax, may be determined. The optimalcapacitance value may, then, be assigned or set to the virtualcapacitor, Cvirtual, and interrogation of the electrosurgical system'soutput impedance (or any other output electrical parameter) maycommence.

Between times t61 and t62 a first interrogation period 840 is initiated.According to the example of FIG. 8, at time t62 it may be determinedthat the therapeutic device controlled by the electrosurgical systemtouches an organ or tissue. (The determination may be made if the valueof the monitored output impedance is, in this example, lower than a‘start impedance’, Zstart, threshold value that may be, for example, 2.2KΩ.) Therefore, the signal generator may start delivering therapeuticenergy (850) to the tissue at time t62. At time t63, it is determinedthat the therapeutic device does not touch the organ or tissue. (Thedetermination may be made if the value of the monitored instantaneousoutput impedance is, in this example, greater than a ‘stop impedance’,Zstop, threshold value that may be, for example, 3.5 KΩ.) Therefore, thesignal generator stops delivering the therapeutic energy (860) to thetissue at time t63.

At time t64, tissue contact is detected again (through monitoring of theinstantaneous output impedance). (The determination is made if the valueof the monitored instantaneous output impedance is, again, lower thanthe start impedance threshold value, Zstart.) Therefore, the signalgenerator restarts delivering therapeutic energy (870) to the tissue attime t64.

At time t65 it is determined that the instantaneous output impedanceindicates that the electrosurgical system's output impedance is in openstate, in which condition another cable interrogation process 880 maycommence, for example, to find a more suitable capacitance value for thevirtual capacitor. (‘More suitable’ means able to accommodate for achange in the electrical properties (e.g., leakage impedances) of theelectrosurgical system, or part thereof.)

During period 816, a series of interrogation periods may be used tomonitor the electrosurgical system's instantaneous output impedance.Interrogation of an instantaneous output impedance may be performedduring a period when the signal generator does not deliver therapeuticenergy. That is, delivering of the therapeutic energy to an organ ortissue may be pulsed (e.g., in bursts), and the interrogation of theinstantaneous output impedance may be performed in-between suchpulses/bursts.

The articles “a” and “an” are used herein to refer to one or to morethan one (i.e., to at least one) of the grammatical object of thearticle, depending on the context. By way of example, depending on thecontext, “an element” can mean one element or more than one element. Theterm “including” is used herein to mean, and is used interchangeablywith, the phrase “including but not limited to”. The terms “or” and“and” are used herein to mean, and are used interchangeably with, theterm “and/or,” unless context clearly indicates otherwise. The term“such as” is used herein to mean, and is used interchangeably, with thephrase “such as but not limited to”.

Embodiments of the invention may include a computer or processornon-transitory storage medium, such as for example a memory, a diskdrive, or a USB flash memory, encoding, including or storinginstructions, e.g., computer-executable instructions, which whenexecuted by a processor or controller, carry out methods disclosedherein. Having thus described exemplary embodiments of the invention, itwill be apparent to those skilled in the art that modifications of thedisclosed embodiments will be within the scope of the invention.Alternative embodiments may, accordingly, include more modules, fewermodules and/or functionally equivalent modules. The present disclosureis relevant to various types of electrosurgical systems (e.g., autobipolar type electrosurgical systems, monopolar type electrosurgicalsystems, etc.), to various types of cables (e.g., auto bipolar cables,etc.), and to various types of electrosurgical devices. Hence the scopeof the claims that follow is not limited by the disclosure herein to anyparticular electrosurgical system or electrosurgical device.

1-17. (canceled)
 18. An electrosurgical system comprising: anelectrosurgical instrument; and an electrosurgical generator coupled tothe electrosurgical instrument through a cable, the electrosurgicalgenerator including: a signal generator configured to output aninterrogatory signal to interrogate the electrosurgical generator, theelectrosurgical instrument, and the cable; and a controller configuredto: assign to a virtual capacitor representing leakage capacitance ofthe electrosurgical system a capacitance value; change the capacitancevalue of the virtual capacitor across a series of capacitance values inresponse to the interrogatory signal; calculate a series of outputimpedances respectively corresponding to the series of capacitancevalues; determine a maximum output impedance from the series of outputimpedances; and determine the capacitance value for the virtualcapacitor corresponding to the maximum output impedance.
 19. Theelectrosurgical system according to claim 18, wherein the controller isfurther configured to calculate the series of output impedances bydetermining an output voltage and an output current of the generatorbased on the interrogatory signal.
 20. The electrosurgical systemaccording to claim 19, wherein the controller is further configured tocalculate from the output voltage and the output current, an electricalpower delivered by the generator.
 21. The electrosurgical systemaccording to claim 18, wherein the signal generator is furtherconfigured to output apply an input voltage as the interrogatory signaland the controller is configured to measure an input current in theelectrosurgical generator, the electrosurgical instrument, and the cablein response to the applied input voltage.
 22. The electrosurgical systemaccording to claim 21, wherein the controller is further configured tochange the capacitance value of the virtual capacitor across the seriesof capacitance values, includes for each value of the virtual capacitor:multiplying the input voltage and the input current to obtain an outputvoltage and an output current for the generator; and calculating theoutput impedance of the generator based on the output voltage and theoutput current.
 23. The electrosurgical system according to claim 18,wherein the controller is further configured to determine the maximumoutput impedance from the series of output impedances by interpolatingthe maximum output impedance from the series of output impedances. 24.The electrosurgical system according to claim 18, wherein thecapacitance value of the virtual capacitor is from 50 pF to 600 pF. 25.The electrosurgical system according to claim 18, wherein the controlleris further configured to determine whether the electrosurgicalinstrument is connected to the cable.
 26. An electrosurgical systemcomprising: an electrosurgical instrument; and an electrosurgicalgenerator coupled to the electrosurgical instrument through a cable, theelectrosurgical generator including: a signal generator configured tooutput an interrogatory signal to interrogate the electrosurgicalgenerator, the electrosurgical instrument, and the cable; and acontroller configured to: sweep the interrogatory signal to obtain aseries of capacitance values; calculate a series of output impedancesrespectively corresponding to the series of capacitance values;determine a maximum output impedance from the series of outputimpedances; and determine an optimal capacitance value corresponding tothe maximum output impedance; and assign the optimal capacitance valueto a virtual capacitor.
 27. The electrosurgical system according toclaim 26, wherein the controller is further configured to calculate theseries of output impedances by determining an output voltage and anoutput current of the generator based on the interrogatory signal. 28.The electrosurgical system according to claim 27, wherein the controlleris further configured to calculate from the output voltage and theoutput current, an electrical power delivered by the generator.
 29. Theelectrosurgical system according to claim 28, wherein the signalgenerator is further configured to output apply an input voltage as theinterrogatory signal and the controller is configured to measure aninput current in the electrosurgical generator, the electrosurgicalinstrument, and the cable in response to the applied input voltage. 30.The electrosurgical system according to claim 21, wherein the controlleris further configured to change the capacitance value of the virtualcapacitor across the series of capacitance values, includes for eachvalue of the virtual capacitor: multiplying the input voltage and theinput current to obtain an output voltage and an output current for thegenerator; and calculating the output impedance of the generator basedon the output voltage and the output current.
 31. The electrosurgicalsystem according to claim 26, wherein the controller is furtherconfigured to determine the maximum output impedance from the series ofoutput impedances by interpolating the maximum output impedance from theseries of output impedances.
 32. The electrosurgical system according toclaim 26, wherein the optimal capacitance value of the virtual capacitoris from 50 pF to 600 pF.